If you are able, remove brake pads.
Clean up all of the pads with "brake cleaner", eBay item number:303735494828
Coat rear surface & end parts of pads with MINTEX ANTI-BRAKE SQUEAL GREASE CERATEC 75ml
eBay item number:183877106947
Do not get the grease on the surface of the pads.
While you are at it, spray the cleaner onto the discs & wipe off with a clean rag. (while the pads are out)